1. Field of the Invention
This invention relates to the field of threaded fasteners. More particularly, the invention pertains to a pilot or lead located at the end of the fastener and adjacent screw threads that prevent cross threading when the fastener is threaded into an internally threaded receptacle such as a nut.
2. Description of the Prior Art
Generally, there is no difficulty in starting a threaded screw or bolt in a tapped hole or nut when the two mating parts have their longitudinal axes aligned. However, when the bolt enters the threaded part misaligned angularly with respect to the axis of the threaded nut, the threads may initially engage incorrectly. Thereafter, as the bolt is driven toward full engagement, the threads of the bolt and/or nut fail by shearing. This condition is called cross threading.
When the axis of the bolt or screw is excessively misaligned angularly with respect to the longitudinal axis of the nut, the operator readily perceives misalignment because the threads failed to engage. Generally, corrective action is taken to align the bolt with the nut. However, when misalignment is slight, the threads will engage on the bolt surface that first contacts the nut thread, but the bolt thread diametrically opposite the first threads engage the nut thread in such a way that cross threading results. In severe cases, the bolt is removed from the hole and the nut threads are retapped or the nut is replaced. When cross threading occurs in a modern assembly plant during repetitive operations at relatively high speed, time lost to correct cross threading is unacceptable. Costs associated with correcting this condition are correspondingly prohibitive.
Furthermore, cross threading occurs more frequently when portable power tools are used to drive the threaded fastener as is the case in a modern assembly plant. To avoid this difficulty, frequently the operator starts by hand the bolt threads into the nut to assure proper engagement of the first few bolt threads with those of the nut. Then, a pneumatic power source is attached to a socket that engages a hexagonal head on the end of the bolt. When conventional fasteners are used and angular alignment of the bolt is excessive, potential for cross threading is sensed by the operator when a bolt wobbles at the entrance of the threaded nut but will not engage. When this condition is observed, the operator attempts to realign the fastener and nut before power is again applied to drive the fastener to a fully engaged position.
Often the outermost threads of the nut become clogged with paint and other contaminants before the fastener is inserted. When accumulation of contaminants is large, proper threaded engagement is made more difficult and can induce cross threading. To avoid these difficulties, threaded fasteners have been formed with a lead end, which provides space into which the contaminants may accumulate after removal as the radially outer portions of the threaded lead engage the nut threads U.S. Pat. Nos. 3,681,963; 4,042,342; 4,235,149 and 2,703,419 describe threaded bolts and screws of this type.
U.S. Pat. No. 4,630,985 describes a self-aligning screw intended specifically to avoid cross threading. This screw contains a projection that extends radially from a tappered end such that when the axis of a screw is angularly misaligned with the axis of the internal threads of the nut, the projection makes first contact with the threads on the tapped hole. As the screw is turned, the projection follows the tapped threads of the hole and forces the screw to a position of substantial alignment with the axis of the nut. This projection is unable to align the fastener because no moment is developed to bring the screw axis into alignment with the nut.
Various asymmetrical pilot or lead portions extending from a threaded shank have been developed for use in self-tapping screws and bolts that cut threads in the part engaged by the screw. The pilot or lead surfaces are generally noncircular and result from use of inclined surface directed diagonally with respect to the axis of the shank. Examples of this technique are described in U.S. Pat. Nos. 3,942,406; 4,069,730; 4,534,690 and Australian patent No. 120,691.
A self-tapping screw having a cylindrical lead and a conical transition section located between a cylindrical shank and the lead is described in U.S. Pat. No. 3,724,315. The lead has a noncircular cross section and may have a drill point to facilitate cutting a hole in sheet metal or other material. Partial threads are formed on the conical section. Because this screw is self-tapping, i.e., it forms its own thread while being driven, it cannot cross-thread.
U.S. Pat. No. 4,789,288 describes a frusto-conical bolt lead having its axis parallel to, and offset from, the axis of the shank. A transverse cross section, perpendicular to the shank axis, taken at any point along the conical lead is a circle. The bolt includes one set of partial threads located at the end of the shank adjacent the lead, and a threadless surface diametrically opposite the partial threads.